It is known, for example, from European Patent Specification No. A-0 084 383 for material to be comminuted to be subjected first to individual grain crushing and material bed crushing in a roll crusher under high pressure with agglomeraters being formed. Then the material is sifted, and the grit produced in the sifting is further comminuted in a mill which is arranged downstream from the crushing apparatus.
According to another known arrangement as set out in German Patent Specification No. A-33 02 176 the agglomerates which form in the roll crusher can also be loosened in an arrangement such as a ball mill, dispersing plate air sifter, or grading screen, arranged after the roll crusher, in which use is made of the disagglomerating effect of a device which is primarily associated with another stage in the process. Thus, if the loosening of the agglomerates is to be carried out for example in a conventional dispersing plate air sifter this creates an additional load on the dispersing or delivery plate of the air sifter, so that quite unsatisfactory loosening of the agglomerates must be expected.